Human origins and ancient human DNA

The recent commentary by Constance Holden (Science 12 Jan., p. 230) on the challenge to the ?Out of Africa? theory of modern human origins posed by putative ancient Australian human mitochondrial DNA sequences overlooked several important problems. Ancient DNA discoveries are easily contaminated and carry a considerable burden of proof, especially when they involve human sequences or surprising examples of preservation. Both concerns apply in the case of the ancient Australian remains (up to 60 kyr old), as DNA is not expected to survive for this length of time outside of cold environments and similar remains elsewhere have not yielded genetic material.